Glycerides are fatty acid esters of glycerol. Those in which all three of the glycerol hydroxyl groups are esterified are called triglycerides. Such compounds account for most of the substance of vegetable and animal fats. When only one or two of the hydroxyl groups are esterified with fatty acid, the resulting glycerides are called monoglycerides and diglycerides, respectively. Mono- and diglycerides occur in only a small quantity in the fats synthesized in nature, but are the intermediate products of the hydrolysis of fats. Both long and short chain mono- and diglycerides are known mainly as laboratory products prepared for research or as products manufactured for man's use. Richardson, A. S., "Glycerides," The Encyclopedia of Chemistry, (Hampel and Hawley eds. 1973).
There have been few reports concerning the biological activity of monoglycerides, although isolated references to biological activities of such compounds do exist. For example, Birkhahn et al., demonstrated the possibility of using butyric acid, in the water soluble form of monobutyrin, as a source of intravenous nutrition by infusing the monoglyceride into rats. The American Journal of Clinical Nutrition, 2078-2082 (December 1977). This study showed that the animals could tolerate very high infusions of monobutyrin, up to 27 gm/kg a day, with little or no resulting toxicity and that the compound could provide calories for the animals when infused intravenously.
Another group experimentally demonstrated that monoglycerides having C5-C12 alkyl side chains, especially glyceryl monocaprylate, can be used to enhance the permeability of skin to certain water soluble chemicals and drugs. Okumura et al., Drug Design and Delivery, 6:137-148 (1990).
In 1990, Dobson et al described 1-butyryl-glycerol (monobutyrin) as a novel angiogenic compound that is synthesized naturally and secreted during the differentiation of 3T3-F442A preadipocytes into adipocytes. Angiogenesis refers to the growth of new blood vessels and involves the growth of capillaries composed of endothelial cells. Monobutyrin was shown to promote angiogenesis in the chick chorioallantic membrane assay and motility of isolated endothelial cells in vitro. These results, coupled with an observed 200-fold increase in monobutyrin levels during the adipose differentiation process, suggested that monobutyrin is an important regulatory factor in the development of adipose tissue vasculature. Cell, 61: 223-230, (Apr. 20, 1990).
The discovery of monobutyrin as a naturally occurring angiogenic factor resulted in the filing of a patent application, U.S. Ser. No. 07/327,314, directed to the use of monobutyrin and other short chain monoglycerides of the formula CH.sub.2 OHCHOHCH.sub.2 OCOR, wherein R is a straight or branched chain hydrocarbyl having 2-10 carbon atoms, saturated or unsaturated, in stimulating angiogenesis, especially for wound healing.
In later work by this group, which includes several of the inventors herein, it was shown that monobutyrin production increases substantially at the onset of lipolysis and that monobutyrin production is surprisingly cell specific. Conditioned medium from several other cell lines representative of actively metabolizing tissue failed to reveal detectable levels of monobutyrin. This cell specificity, coupled with knowledge of other bioeffecting molecules having multiple functions in vivo, led the group to consider that monobutyrin might be a pleiotropic effector having multiple functions in blood vessel biology. Because vessels in adipose tissue are known to vasodilate during lipolysis, a correlative hypothesis was made that monobutyrin could be involved in vasodilation and/or vascular permeability. Wilkison et al, The Journal of Biological Chemistry, 25:16886-16891 (September, 1991). The hypothesis was not supported by experimentally, and indeed, was nothing more than conjecture at the time. It is well known from the literature that the concentration of other factors such as glycerol, which do not have vasodilatory activity, also increases after the onset of lipolysis, making it more likely than not that the hypothesized correlation would not be substantiated. It is believed that short chain monoglycerides have not otherwise been associated with vasodilatory activity
There is at least one report in the literature of a long chain monoglyceride, monoolein (C18:1), having a depressor effect on blood pressure. Muirhead et al., The American Journal Of The Medical Sciences, 298:2 93-103 (August 1989). The vasodilatory capability of monoolein was reported to be dependent upon hepatic activation, as evidenced by a period of latency between intravenous infusion of the compound and observation of the depressor effect of the compound and a loss of activity upon removal of the liver. In addition, free fatty acids, particularly caprate, a C10 fatty acid, have been associated with vasodilatory activity. White et al., Stroke, 22:4, 469-476 (April 1991); White et al., Gen. Pharmac., 22:4, 741-748 (1991). The studies by White et al., suggest that saturated fatty acids have vasodilatory activity and that the medium chain fatty acids are far more potent than either the short chain or the long chain ones. By rank, C10 was the most potent, while the C.sub.4 compounds were not particularly vasoactive. The effects of the fatty acids appeared to be independent of the endothelium.
Vasodilation is the dilation of vessels, generally resulting in increased blood flow to a part of the body. A wide variety of vasodilator drugs are known and have been used successfully in the treatment of pathophysiological diseases such as hypertension, angina pectoris, and congestive heart failure, to name a few. These agents may be classified according to their primary mechanism of action. One important group of vasodilators, which includes the nitrates and sodium nitroprusside, exert a direct effect on smooth muscle. Sodium nitroprusside acts to directly relax vascular smooth muscle in both arterioles and veins, while the nitrates, such as nitroglycerin, act principally on the venous bed and pulmonary arteries. Another important group of vasodilatory compounds, which includes captopril, enalapril and lislinopril, appear to exert their activity through the inhibition of enzymatic conversion of angiotensin I to angiotensin II, which is a potent constrictor of arteriolar resistance vessels. Alpha and beta-adrenoreceptor blocking agents and calcium antagonists have also been used successfully as vasodilators.
Despite reports of the development of pharmaceutical agents that lower blood pressure, improve congestive heart failure, or hasten recovery from anginal episodes, none of the vasodilator drugs currently available is ideal. A need continues to exist for medicaments that are useful in the treatment of these disorders, and especially for medicaments that not only exhibit a desirable pharmacological profile, but are also non-toxic, do not induce tachyphylaxis, and are inexpensive to manufacture. A need also exists for vasodilators that have a localized vasodilatory effect which can be used to counteract disorders associated with vasoconstriction in localized or regional vascular beds.
It is an object of the invention to provide short and medium chain monoglycerides capable of stimulating vasodilation in arm-blooded animals that are highly non-toxic and suitable for pharmaceutical formulation and administration.
It is another object to provide vasodilatory compounds that have localized vasodilatory effects.